Magnetometer sensors making use of negatively charged nitrogen-vacancy centers (NV−) in diamond are known from patent publications U.S. Pat. No. 8,547,090 and WO 2014/011286. Those magnetometers make use of the specific spin states of NV− centers, the ground state of NV− being a triplet state whose sub-levels are split in energy into a singlet state of spin projection ms=0 and a doublet ms=±1 and the energy separation is 2.87 GHz in absence of a magnetic field. These patent publications disclose the use of a light source (typically a green laser beam) to excite the ground state of NV− to the NV− excited state which is also a spin triplet. The excited state has a spin-dependent probability of either returning to the ground state with a red-shifted photoluminescence (PL) or decaying non-radiatively through a singlet and metastable state. Those magnetometers are read optically, by detecting emitted photons induced by electron transitions on the NV state. The excited states with spin projection ms=±1 have a higher probability of following the non-radiative path compared to the excited ms=0 state. By applying a microwave field at resonance frequency the ms=±1 ground state can be favorably populated. As a result, when varying the microwave frequency over the resonance, the photoluminescence is decreased and a typical electron spin resonance spectrum (ESR) is obtained. From this photoluminescence spectrum, as discussed in the above mentioned patents, information with respect to the presence or orientation of a magnetic field can be deduced (Zeeman splitting).
Applications for detecting weak magnetic fields using magnetometers based on nitrogen-vacancy centers in diamond are numerous and are for example described by Rondin et al in “Magnetometry with nitrogen-vacancy defects in diamond”, Rep. Prog. Phys. 77 (2014).
A drawback of these optically read magnetometers and methods disclosed (e.g. U.S. Pat. No. 8,547,090 and WO 2014/011286) is that they require a complex optical setup and expensive single photon detector to detect the PL light. Moreover, those optical detection systems have a poor light collection efficiency. These optical setups for reading out the PL light also results in a limited spatial resolution for detecting magnetic fields. With currently available technology and using a sophisticated optical design, the spatial resolution is still limited to about 300 nm.
With the ODMR technique the signal to noise ratio is also limited by the shot noise.
More recently, the inventors of the current patent application, announced a technique using a photocurrent measurement rather than an optical measurement to detect electron spin resonances in NV− centers. The principles of this technique were published in February 2015 on arXiv.org, reference 1502.07551 by E. Bourgeois et al., “Photoelectrical detection of electron spin resonance of nitrogen-vacancy centers in diamond”. This technique, called PDMR (Photocurrent Detection of Magnetic Resonance) is based on the detection of charge carriers promoted to the conduction band of diamond by two-photon ionization of NV− centers. Using an electrode, charges are collected and a photocurrent is measured. As illustrated in FIG. 1, the intensity of photoluminescence and photocurrent were measured simultaneously while scanning the microwave frequency, in the absence and in the presence of an external magnetic field. The photocurrent I measured with the PDMR technique is expressed in nA (nano Ampere) while the measurement of the PL light using the ODMR technique is expressed as the number of photons measured per second. Similar results were published by E. Bourgeois in Nature Communications 6, 8577 (2015).
In FIG. 1, the curve 34, representing the measured photocurrent as function of microwave (MW) frequency, is a curve obtained without applying an external magnetic field while curves 35 and 36 are obtained by applying an external magnetic field along the [100] and [111] crystal direction, respectively. In this example the external magnetic field applied was 0.5 mT for the [100] direction and 2.0 mT for the [111] crystal direction. When comparing the curves 34, 35, 36 obtained with the magnetometer according to example embodiments (PDMR technique) with the curves 31, 32, 33 obtained through an optical technique (ODMR) under the same magnetic field conditions, the splitting and the minima in the curves are observed at the same frequencies for both techniques. This principle offers the possibility to read the NV spin state that can be used in quantum applications.
However, as discussed by E. Bourgeois et al. in the above mentioned publication, a major difference between the PDMR and ODMR technique is that the observed contrast value of the ESR spectrum is much lower with the PDMR technique. This is due to the photoionization of nitrogen and other defects in diamond that contribute to the photocurrent. The contrast being defined as the percentage of photocurrent reduction at the resonance frequency compared to the photocurrent off-resonance. The maximum contrast value observed for a NV ensemble by PDMR in referred publication was about 1% and with ODMR it was 10.7%. As discussed by Rondin, Rep. Prog. Phys. 77 (2014), the sensitivity of a magnetometer, defined as the minimum magnetic field detectable, is inversely proportional with the contrast value and the total counts. Hence there is room for improving the PDMR technique for use as a magnetometer sensor.